1. Humboldt-Universität zu Berlin, Department of Physics, Brook-Taylor-Straβe 6, D-12489 Berlin, Germany
2. Montanuniversity Leoben, Chair of Atomistic Modelling and Design of Materials, Franz-Josef Straβe 18, 8700 Leoben, Austria
3. Johannes Kepler Universität Linz, Institut für Experimentalphysik, Altenbergerstraβe 69, A-4040 Linz, Austria
4. Graz University of Technology, Institute of Solid State Physics, Petersgasse 12, A-8010 Graz, Austria